In the bad old days before XP, this was a frequent occurence on computers which ran on a Windows OS. In some of the worst cases, you only had to look at the computer in a funny manner for the temperamental bastard to give you the Blue Screen Of Death! It usually tells you some shit like "a fatal error has occured" and gives you some mathematical gibberish. Basically, it means the operating system is rather shite and can't be arsed doing what you wanted it to do...

I remember a few years ago on my section at work we had this computer that gave us the Blue Screen Of Death so fuckin' often on a daily basis that I ended up decorating the monitor with the chequered tape we normally use on files if someone is terminally ill!